Hi! I did change my diet post-surgery, but it was because I had facial paralysis that lasted for a while, and I developed a semi-irrational fear of choking. So I avoided meats and other things that required a lot of chewing. But even after my facial function began to return, and I got over the choking fear, I found myself choosing healthier foods because I thought it might aid in the recovery process -- couldn't hurt, right? My doctors encouraged me to take in a lot of protein and vitamins B and E to boost my energy level (I had lost a lot of weight at that point -- most of which I needed to lose). It's been 2+ years now and I'm much better overall, but I still think in terms of "eat to live", or making healthier selections. Pre-AN, I "lived to eat". I haven't gone to extremes; just trying to make better food choices and feeling better for it.
On a similar note, if I could have one do-over, I would have taken an extra few months prior to surgery to really get in shape. I work a desk job, was carrying about thirty extra pounds, and was not physically active. I ended up with some balance issues, and have worked hard since to develop and maintain my abdominal core strength. The physical therapist started me on an exercise regimen, and I continued with Pilates afterwards. I think if I had been in physically better shape at surgery, my recovery might have been easier. But that's just my opinion. Every case is different, and chances are you will not experience those issues. But I encourage you to continue thinking in terms of self-care. Have no idea if it helps with tumor growth rate, but just knowing you are doing all you reasonably can to improve your physical health seems to give a mental boost as well.
Oh, one more thing. This may be TMI, but they wouldn't release me from the hospital until I had a BM. Why, I don't know; I mean, that's not the end they operated on. But pain killers are notorious for causing constipation. Diet and probiotics can help in that regard.
Hope this helps. I am glad that you have found this forum. There's a lot to take in with this diagnosis, so keep asking questions. All the best to you!